Chirality and Diastereoselection of Δ/Λ-Configured Tetrahedral Zinc Complexes through Enantiopure Schiff Base Complexes: Combined Vibrational Circular Dichroism, Density Functional Theory, 1H NMR, and X-ray Structural Studies

The metal-centered Δ/Λ-chirality of four-coordinated, nonplanar Zn(A∧B)2 complexes is correlated to the chirality of the bidentate enantiopure (R)-A∧B or (S)-A∧B Schiff base building blocks [A∧B = (R)- or (S)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-κ2N,O with X = OCH3, Cl, Br]. In the solid-state the (R) ligand chirality induces a Λ-M configuration and the (S) ligand chirality quantitatively gives the Δ-M configuration upon crystallization as deduced from X-ray single crystal studies. The diastereoselections of the pseudotetrahedral zinc-Schiff base complexes in CDCl3 solution were investigated by 1H NMR and by vibrational circular dichroism (VCD) spectroscopy. The appearance of two signals for the Schiff-base −CHN– imine proton in 1H NMR indicates an equilibrium of both Δ- and Λ-diastereomers with a diastereomeric ratio of roughly 20:80% for all three ligands. VCD proved to be very sensitive to the metal-centered Δ/Λ-chirality because of a characteristic band representing coupled vibrations of the two ligand’s CN stretch modes. The absolute configuration was assigned on the basis of agreement in sign with theoretical VCD spectra from Density Functional Theory calculations.